Study On The Structure Of Carbon Nitride Based Photocatalysts And The Mechanism During Their Photocatalytic Degradation Of Antibiotics Under Visible Light

In the past years,the concentration of antibiotics constantly increases in environment,especially in the water,which is caused by their large-amount use.How to remove these antibiotics efficiently with no pollution is becoming a focus of science research.As a new environmentally-friendly technology,photocatalysis is widely applied in pollutants degradation.Among many photocatalysts,graphitic carbon nitride（g-C₃N₄）receives much interest for its excellent performance and non-metallic character.Therefore,this paper firstly explores the structure of g-C₃N₄ prepared by different precursors and analyzes the cost and performance.Then the g-C₃N₄ is modified with ammonium citrate（AC）and the obtained composite shows more excellent performance in antibiotics degradation.At the same time,its structure is deeply studied and the photocatalytic reaction during degradation is specifically discussed.The details are shown below:（1）Three nitrogen-containing molecules,urea,dicyandiamide（DCD）and melamine are use as precursors to prepare three kinds of g-C₃N₄,which labeled as UCN,DCN and MCN.Experiments results demonstrate MCN>DCN>UCN in the yield value.According to their yield and the commercial price of the corresponding precursors,the calculated results show UCN>DCN>MCN in the cost.After the series of characteristics,it is found that UCN possesses looser and layered structure and wider visible-light–response field.MCN has the lowest photogenerate electrons and holes recombination ratio.When they are used for photocatalytic degradation of tetracycline（TC）,it show that UCN>DCN>MCN in degradation efficiency.Therefore,although DCN was both second in the cost and efficiency,it has the highest efficiency(0.0079 min^-1)/cost（114.2 yuan/500 g）value.So the following experiments choose DCD-prepared g-C₃N₄ as object.（2）Metal-free carbon nitride with controlled carbon dopants（ACCN）is synthesized by one-step copolymerization of DCD and AC.Subsequently series of characteristics are conducted to deeply explore the structure of ACCN.Scanning electron microscope（SEM）and transmission electron microscope（TEM）images demonstrate that ACCN exhibits more regular layered and porous structure.N₂ physisorption measurements demonstrate the addition of AC makes the specific surface area and average pore of ACCN increase to some extent.The results of XRD demonstrate the crystalline structure of ACCN has no obvious changes but become denser.At the same time,fourier transform infrared（FTIR）,organic elemental analysis（OEA）,x-ray photoelectron spectrum（XPS）,thermogravimetric analysis（TGA）and electron paramagnetic resonance（EPR）are conducted to synthetically analyze the structure of ACCN.It demonstrates that carbon atoms replace the bridging nitrogen atoms and some carbon species linked to the introduced carbon atoms are also introduced to the framework of ACCN.On the other hand,the addition of AC contributes to the formation of nitrogen defects during the formation of ACCN.The band structure of ACCN is determined through UV–Vis diffuse reflectance spectra（DRS）and valance band-x-ray photoelectron spectrum（VB-XPS）,which proves it possesses narrower band gap and downshifted conduction band and valence band potential.Therefore,during the measurement on the photoluminescence spectra,transient photocurrent and electrochemical impedance spectra of ACCN,it shows excellent photoelectrochemical characters like higher electrons-holes separation efficiency,lower impedance and faster charge transfer.These indicate ACCN possesses outstanding photocatalytic capacity.（3）The photocatalytic ability of ACCN is studied by its degradation of sulfamethazine（SM2）under visible light（>420 nm）.The results demonstrate that optimal ACCN can degrade 81%of SM2 solution（10 mg/L）in 60 min and the reaction apparent rate constant is0.0277 min^-1,which is 6.6 time that of CN.This indicates the introduced carbon and nitrogen defects contribute to the increased photocatalytic ability of ACCN.At the same time,experiments demonstrate that with the increased concentration of SM2,the removal ratios are decreased.So in practical applications,dilution pretreatment is necessary in the SM2-containing waste water treatment.Furtherly,high performance liquid chromatography-mass spectrometer（HPLC-MS）was used to determine the intermediates in the degradation of SM2.There happens SO₂ extrusion or smile-type rearrangement reaction.The results of total organic carbon（TOC）and three-dimensional excitation-emission matrix fluorescence spectra（3D EEMs）indicate that ACCN possess stronger mineralization ability in the degradation of SM2.The TOC removal ratio achieve at 68%.In addition,through active radicals trapping tests and electron spin resonance（ESR）analysis,the mechanism during the photocatalytic degradation of SM2 is deeply discussed.During the process,?O₂^-and h⁺play the main roles while?OH plays the minor role.At the same time,three-cycle experiments reflect the high stability and durability of ACCN-3.This work provides a facile and green strategy to prepare nonmetal modified CN with boosting photo-redox ability for pollutants treatment and provides deeper theory for its practical application.